Make decl_runtime_apis! fail on methods with default implementation (#7371)

* Make `decl_runtime_apis!` fail on methods with default implementation

Runtime api functions are not allowed to have default implementations.
This fixes this by throwing an error when we detect such a function.

* Update primitives/api/proc-macro/src/decl_runtime_apis.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Update primitives/api/test/tests/ui/no_default_implementation.stderr

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/primitives/api/proc-macro/src/decl_runtime_apis.rs

@@ -912,6 +912,13 @@ impl CheckTraitDecl {
 				.entry(method.sig.ident.clone())
 				.or_default()
 				.push(changed_in);
+
+			if method.default.is_some() {
+				self.errors.push(Error::new(
+					method.default.span(),
+					"A runtime API function cannot have a default implementation!",
+				));
+			}
 		});
 
 		method_to_signature_changed.into_iter().for_each(|(f, changed)| {

substrate/primitives/api/test/tests/ui/no_default_implementation.rs

@@ -0,0 +1,9 @@
+sp_api::decl_runtime_apis! {
+	pub trait Api {
+		fn test() {
+			println!("Hey, I'm a default implementation!");
+		}
+	}
+}
+
+fn main() {}

substrate/primitives/api/test/tests/ui/no_default_implementation.stderr

@@ -0,0 +1,8 @@
+error: A runtime API function cannot have a default implementation!
+ --> $DIR/no_default_implementation.rs:3:13
+  |
+3 |           fn test() {
+  |  ___________________^
+4 | |             println!("Hey, I'm a default implementation!");
+5 | |         }
+  | |_________^